Maro says that he has been tracking the Dark Brotherhood and waiting for an opportunity to attack. On hearing news of the death of their leader, he tasks the Dragonborn with infiltrating the Dark Brotherhood Sanctuary, using the password his agents recently acquired, and slaying every assassin inside.

The assassins can be powerful, and collectively possess a range of attacks that make fighting several of them in the cramped space fairly dangerous. Ironically, although they're assassins, it is particularly easy to sneak up on them. The use of bows, invisibility and poisons are effective ways to take care of them.

When the job is done, report back to Commander Maro to complete the quest and collect the reward of 3,000. Once the quest is complete, the Dark Brotherhood Sanctuary will remain accessible and undamaged, in contrast to finishing the Dark Brotherhood quest line when most of it is destroyed.

I have killed Astrid, the leader of the Dark Brotherhood. I should report her death to a guard.

Objective: Report Astrid's death to a guard

I've spoken to a guard about Astrid's death. He told me I should share this information with Commander Maro, in the village of Dragon Bridge.

Objective: Speak with Commander Maro

Commander Maro has tasked me with killing every assassin in the Dark Brotherhood Sanctuary! He has provided me with the pass phrase that will allow me to enter.

Objective: Kill everyone in the Sanctuary!

I have killed every assassin in the Dark Brotherhood Sanctuary. I should now report back to Commander Maro.

Objective: Report back to Commander Maro

After killing Astrid, the leader of the Dark Brotherhood, I was commissioned by Commander Maro to wipe out the assassins' Sanctuary. I did so, and was rewarded for my efforts. Of course, it will now be impossible for me to join the shadowy organization...

Guards can be heard saying afterwards, "I could've gone into that sanctuary and killed all those Dark Brotherhood types but I was... sick that day."

When the Dragonborn reports Astrid's death to a guard, they say "I killed the leader of the Dark Brotherhood," though there is no way in-game for the Dragonborn to know that she was their leader. If the Dragonborn survives an attack by a Dark Brotherhood Assassin, it will yield a note signed by Astrid, which gives some clue as to her position. Likewise, if the Dragonborn breaks into the room in the Black-Briar Manor basement where Maven Black-Briar keeps an effigy, there is a note to the Brotherhood addressed to Astrid, which further implies her leadership of the Brotherhood.

It is worth noting that completing the Dark Brotherhood questline, plus all of the additional assassinations that Nazir has to offer, yields 20,000, while destroying the Brotherhood earns only 3,000. Other quest rewards, such as Muiri's marriage option and Shadowmere, will likewise be forfeit.

The total amount of coinage that most players will achieve - with the included 20,000 coins, will range anywhere from around 29,000 - 34,000. This is including the side contracts that Nazir offers, and other missions, such as Mourning Never Comes, that are required to complete the questline for the Dark Brotherhood.